In the production of pulp from wood raw material under alkaline conditions by the sulphate method (also called kraft pulping) in which NaOH and Na.sub.2 S are the active cooking chemicals, the fatty acids and rosin acids from wood extractives are saponified and form a soap. In addition to the above organic components, soap also contains "neutral substances", i.e. unsaponifiable organic components, likewise derived from wood extractives.
Softwood contains both fatty acids and rosin acids. Hardwood contains only fatty acids and additionally a greater amount of unsaponifiables than softwood.
It is known that soap can be acidified for example by sulphuric acid treatment. This is currently the predominant method in the chemical pulp industry. The resultant mixture of fatty acids and rosin acids and unsaponifiables is called crude tall oil.
When softwood or a blend of hardwood and softwood is cooked, soap is readily separated in the different stages of black liquor treatment and evaporation. Disturbance-free operation of the pulp mill requires collection and further treatment of the soap.
When the proportion of hardwood increases, the quality of the resultant crude tall oil in view of further refining is impaired, since hardwood yields only fatty acids and unsaponifiables into the crude tall oil. Often tall oil or soap must be added to pure hardwood cooking, in order for the extractive content of the resultant chemical pulp not to be too high. In that case, soap separates also from pure hardwood cooking. If this soap is acidified, it is no longer useful for further refining by the current methods. Said soap must be burned as such, or converted to tall oil and burned.
The quality of tall oil is best illustrated by the acid number. In Finland, plain pine cooking yields a product having an acid number of 160-170. Hardwood cooking yields an oil having an acid number of about 120-130. Oil having an acid number in excess of 145 is suitable for further refining.
Soap having too low an acid number must be burned. This is usually conducted in a soda recovery boiler, thus allowing recovery of the sodium present in the soap. In a lime sludge reburning kiln, sodium normally presents too great problems. On the other hand, burning in a soda recovery boiler requires that the boiler has 5-10% extra capacity, which is usually not available. Thus the soda recovery boiler is often the bottleneck of the production. If the sodium content of soap could be advantageously and sufficiently reduced, burning would also be possible in a lime sludge reburning kiln.
The air pollution control measures to be implemented in the near future will reduce the emission of gaseous sulphur compounds to 1.5 kg of sulphur/ton of chemical pulp. With current technology, emissions from a pulp mill using half softwood and half hardwood amount to 1 kg of sulphur/ton of chemical pulp for processes other than those carried out in the tall oil cooking department. Sulphur is introduced into the circulation of chemicals from different sources, such as wood, water, combustion fuel oil for the lime sludge reburning kiln and magnesium sulphate for oxygen bleaching, in an amount of about 2 kg of sulphur/ton of chemical pulp. This alone produces an excess of sulphur of about 0.5 kg/ton of chemical pulp. It is further to be noted that waste acid from chlorine dioxide plants contains 7-24 kg of sulphur/ton of chemical pulp. Thus the waste acid must be sewered, or an equivalent amount of sulphur must be withdrawn from the recycling by other means.
The above disadvantages have led to a need for a process for producing tall oil without addition of sulphur to the chemical cycle, in which the necessary acidifying chemical is derived from the circulation of chemicals in the pulp mill, in order for future tall oil production and further refining to be possible.
U.S. Pat. No. 3,901,869 suggests as a solution partial acidification of soap with carbon dioxide and final acidification with sulphuric acid or with waste acid from a chlorine dioxide plant. Yet it has showed that as a result of such a procedure, too much sulphur still remains in the circulation of chemicals, and further investment costs mount up.
In accordance with U.S. Pat. No. 2,430,029, soap is converted to tall oil by a two-step process, wherein partial conversion is first conducted with a sodium bisulphite solution, i.e. used brine from the second step, and the final acidification is performed with gaseous SO.sub.2.
In a modern chemical pulp mill, the concentrated and dilute sulphur-containing odour gases that are produced must be collected and burned for environmental reasons alone. Burning is normally conducted in a separate burner or in a lime sludge reburning kiln. In both cases, dilute gases containing sulphur dioxide are obtained, and the sulphur contained in these gases is absorbed into NaOH. Alternatively, the procedure may involve absorption of the SO.sub.2 needed for the post-acidification of the soap into fresh (make-up) NaOH, and absorption of the remaining SO.sub.2 into oxidized white liquor, the resultant NaHSO.sub.3 solution being used for initial acidification of the soap. The SO.sub.2 needed for the post-acidification is prepared from NaHSO.sub.3 solution obtained by absorption into NaOH.sub.3, by evaporation whereupon the NaHSO.sub.3 is decomposed into SO.sub.2 and NaHSO.sub.3.
The concentration of the NaHSO.sub.3 solution obtained from absorption is about 20%. The amount of sulphur in this solution is about 2-3 kg/ton of chemical pulp. If the mill has a liquor heat treatment (LHT) process in which the viscosity of black liquor is reduced by thermal treatment, the amount of sulphurous gases produced therein may be increased so that the total sulphur flow is as high as 10 kg of sulphur/ton of chemical pulp.
As set out above, the sodium bisulphite solution resulting from SO.sub.2 absorption in combustion of odour gases cannot be included in the circulation of chemicals within the framework of the future environmental regulations.